Two weeks ago I stood in a dry stream bed at Castle Eden Dene, wondering at the absence of water yet also conscious that many of the stones that littered the surface had a slipperiness that suggested not only that they had been wet relatively recently, but also that the surface biofilms (which impart this slipperiness) might still be intact. A first look at a portion of this film under my microscope suggested that this might well be the case: I could certainly see some diatoms, and some green algae cells, but most were very small and that there was also a lot of particles, both inorganic and organic, that made viewing these algae quite difficult. Since then, I’ve prepared a permanent slide from this material, so I can now take a closer look and get a better idea of what diatoms thrive in a dry stream bed in mid-winter in northern England.
A quick analysis of the sample found 34 species, of which four were abundant (comprising over 60% of the total) and the remainder were relatively infrequent. The most abundant species was Amphora pediculus, which I’ve written about before, and which was not a surprise, as it is a species that thrives in the hard water that I would have expected in a stream draining a limestone catchment. The other three common species wereHumidophila contenta, Luticola muticaand Simonsenia delognei, all of which are known to survive in habitats that are not permanently submerged. These are relatively uncommon in the typical samples that I encounter but when they do occur in large numbers, they are often found together. It is another facet of the “London Bus” paradigm that I described in the previous post, except this time it is a characteristic assemblage of species from different genera, rather than from a single genus or family.
Some of the diatoms from Castle Eden Burn, January 2019: a. Nitzschia nana; b. – g. Luticola mutica; h. – k. Humidophila contenta. Scale bar: 10 micrometres (= 1/100thof a millimetre).
Diatoms in the genus Humidophilahas changed names twice over the course of my career. Back in the 1980s, species from this genus, as well as Luticolawere considered to be part of the Navicula which was regarded as a “dump for all bilaterally symmetrical [e.g. boat-shaped] raphid diatoms lacking particularly distinctive features” according to Frank Round, Dick Crawford and David Mann. They split several groups of species away from Naviculato create new genera, one of which was Luticola. In other cases, to resurrect old genera that had been subsumed into Naviculain the first half of the 20thcentury. One of these resurrected genera was Diadesmiswhich differed from “true” Naviculain several respects, not least of which was a tendency to form ribbon-like colonies. A more recent study suggested that Diadesmis, itself, needed to be split, with several species being moved to yet another new genus, Humidophila. Unfortunately, the criteria on which this was based are not easily seen with the light microscope. However, one by-product of this split was that all the species within the genera that are associated with damp, rather than fully-submerged habitats, ended up in the new genus rather than in Diadesmis. That lends weight to the split, suggesting that there is more to the separation than just minor differences in the details of the cell wall.
The final species that was common in Castle Eden Burn was Simonsenia delognei. This is another small diatom and, as I could not get good photographs from this sample, I have included photographs from another site to show what it looks like. It is a very delicate diatom, easily overlooked when scanning a slide, particularly as it usually only occurs in small numbers. That, again, might be because I usually look at samples from fully-submerged habitats. Here, it formed about 12 per cent of the total number of valves, which is four times as many as I have previously found.
Simonsenia delogneifrom Ballyfinboy River, Co. Tipperary, August 2014. Scale bar: 10 micrometres (= 1/100thof a millimetre). Photographs: Lydia King.
I’m quite intrigued, now, to see how the algal communities change over the course of the year. Are these diatoms that can tolerate drying ever-presents or will their proportions fluctuate over the course of the year as the stream comes and goes? And what is it that makes some diatoms cope with these dry periods? The ability to live out of water is associated with a few genera in particular, so what is it about their genetic make-up that lets them thrive. What about Amphora pediculusand the other diatoms that I associate with submerged habitats? Am I looking at dormant but viable cells (I did not see many healthy chloroplasts when I made my initial observations) or are these diatom carcasses strewn across an arid desert? At the risk of sliding into metaphor-overload, does this mean that Humidophila, Luticolaand Simonseniaare the cacti of the diatom world?
Lowe, R.L., Kociolek, P., Johannsen, J.R., van de Vijver, B., Lange-Bertalot, H. & Kopalová, K. (2014). Humidophilagen. nov., a new genus for a group of diatoms (Bacillariophyta) formerly within the genus Diadesmis: species from Hawai’I, including one new species. Diatom Research29: 351-360.
Round, F.E., Crowford, R.M. & Mann, D.G. (1990). The Diatoms: Biology and Morphology of the Genera. Cambridge University Press, Cambridge.